There are many systems where the isolation of specific cells from a larger number or population of cells is highly desirable. For example, circulating tumor cells (CTC's) are cancer cells that have been displaced from their associated tumors and may enter the blood system or other parts of the vasculature. CTC's may offer information with respect to the condition and progress of a cancer; they may also serve to cause cancerous growths at other points within a body, e.g., at locations where there was no cancerous legion prior to the arrival and plastic growth of the CTC's. Circulating fetal cells (CFC's) are similar to CTC's in that they are often found in very low concentration in the blood; their obvious source is from a developing fetus in the mother's womb. CFC's may offer a rich potential for genetic analysis of a fetus at very early stages of fetal development.
Because of the importance of CTC's and CFC's, their determination and/or isolation and use in genetic and medical analyses is extremely valuable. The major challenge with these cell groups is their low circulating numbers: there may be a few tens of such cells per milliliter of blood, as opposed to the millions of white blood cells and the like, and many millions of red blood cells.